Since aliphatic polyester resins such as polyglycolic acid resins (sometimes called a “PGA” hereafter) or polylactic acid resins (sometimes called “PLA” hereafter) are degraded by microorganisms or enzymes existing in the natural world such as in soil or in oceans, attention has been focused on these resins as biodegradable polymer materials with a small burden on the environment. In addition to the biodegradability, these aliphatic polyesters have hydrolyzability and use of the aliphatic polyesters in various fields has been actively investigated in recent years.
Among aliphatic polyester resins, a PGA has excellent mechanical strength as well as excellent gas barrier properties such as oxygen gas barrier properties, carbonic acid gas barrier properties, and water vapor barrier properties or aroma barrier properties. Since a PGA is a heat-resistant material which has a high melting point and can be melt-molded, the applications of PGA as a biodegradable and hydrolyzable resin with excellent practicability are expanding, either with PGA alone or as a composite with other resin materials or the like. PGA is used as a molding material for forming molded products by means of general-purpose resin molding methods such as injection molding, extrusion molding (including solidification- and extrusion-molding), compression molding, and blow molding. For example, PGA is used as a material for forming molded products (specifically, film molded products or the like) such as packaging materials for food products or the like which are susceptible to oxidative degradation, or easily compostable packaging materials having a small environmental burden.
Furthermore, a PGA is increasingly anticipated as a downhole tool member for hydrocarbon resource recovery that is, a downhole tool member for hydrocarbon resource recovery (also simply called a “downhole tool member” hereafter) which can be left underground and degraded after use by taking advantage of the strength and degradability (biodegradability and hydrolyzability) of the PGA.
In order to recover hydrocarbon resources such as petroleum (including shale oil) or natural gas (including shale gas) from underground, a downhole for forming a well such as an oil well or a gas well is provided. Drilling includes, for example, the operation of expanding the amount of petroleum or natural gas produced by forming a shaft by drilling into the subterranean formation with a drill while circulating mud water and generating holes or cracks in the productive layer by using a tool containing gunpowder such as a perforation gun or by injecting fracturing fluid into the subterranean formation at a high pressure (fracturing). In order to form or repair a downhole, a tool such as a frac plug, a bridge plug, a cement retainer, a perforation gun, a ball sealer, an isolation plug, or a packer—that is, a downhole tool for hydrocarbon resource recovery (also simply called a “downhole tool” hereafter) is used, ordinarily in plurality, and disposed inside the downhole. Rather than being retrieved to the ground after use, the downhole tool has often been disposed by disintegrating or dropping the tool directly inside the downhole. Therefore, a member constituting the entire downhole tool or a coupling site for promoting disintegration (corresponding to a downhole tool member) is formed from a degradable material such as a degradable polymer (Patent Documents 1, 2, and the like). Examples of degradable polymers include polysaccharides such as starch and dextrin; animal proteins such as chitin and chitosan; aliphatic polyesters such as PLA (a typical example of which is poly-L-lactic acid (PLLA)), PGA, polybutyric acid, and polyvaleric acid; polyamino acids, and polyethylene oxide.
As the depth of well drilling increase, the temperature and pressure of the downhole environment become increasingly high. There are increasing expectations for PGA, which has excellent mechanical strength, degradability, and heat resistance under temperature conditions of, for example, 66° C. (equivalent to 150° F.), 80° C., 93° C., 121° C., and 149° C. (equivalent to 300° F.) as well as temperatures near 200° C. That is, PGA has excellent degradability and can degrade in a desired short period of time, even in the form of a molded product having a large effective thickness (referring to the maximum thickness or maximum diameter of the molded product).
On the other hand, as well drilling becomes increasingly diverse, downhole environments with lower temperature conditions than temperature conditions of conventional environments have emerged. It is desired that PGA, which has excellent degradability in downhole environments with a temperature of 66° C. or higher, for example, will also have excellent degradability and can be degraded in a desired short amount of time even in relatively low-temperature downhole environments such as a temperature of lower than 66° C. That is, a molded product having a small effective thickness (which is equivalent to a small thickness or a small diameter) can be degraded in a short amount of time, but there have been hopes for a molded product having an effective thickness of 1 mm or more such as a downhole tool member, for example, which can also be degraded in a desired short period of time.
The present inventors proposes a polyester resin composition containing 100 parts by mass of a polyester resin containing not less than 50 mass % of a glycolic acid resin and from 0.5 to 50 parts by mass of a carboxylic acid anhydride as a polyester resin composition for forming a molded article of a shape such as a powder, a pellet, a film, or a fiber to be compounded and used in a well treatment fluid such as a fracturing fluid (Patent Document 3). In Patent Document 3, a powder having a major/minor axis ratio of not greater than 1.9 and a cumulative 50 wt. % average diameter of 1 to 1,000 μm, a pellet having a length of 1 to 10 mm in the longitudinal direction and an aspect ratio of not less than 1 and less than 5, a film having an area of 0.01 to 10 cm2 and a thickness of 1 to 1,000 μm, and a short fiber having a length/cross-sectional diameter ratio (aspect ratio) of 10 to 2,000 and a minor axis of 5 to 95 μm are given as examples of molded articles. Patent Document 3 discloses that these fibers are contained in a fracturing fluid at a concentration of 0.05 to 100 g/L and preferably from 0.1 to 50 g/L. However, there is no suggestion in Patent Document 3 of a molded product having an effective thickness of 1 mm or more such as a downhole tool member.
Therefore, there has been a demand for a molded product containing an aliphatic polyester resin, which is a degradable material, and having an effective thickness of 1 mm or more, having excellent degradability even in relatively low-temperature downhole environments such as a temperature of lower than 66° C., for example, so as to be degradable in a desired short amount of time, and having sufficient mechanical strength.